This invention relates to a solder paste for forming a tin-bismuth solder connection having enhanced high temperature properties. More particularly, this invention relates to a solder paste that comprises a mixture of tin-bismuth alloy powder and a powder of a tertiary metal, preferably gold or silver, effective to enhance the high temperature properties of the product connection.
Solder paste is employed to form a solder connection, for example, for surface mounting a component to a printed circuit board or the like. A typical paste comprises a powder formed of a solder alloy and dispersed in a liquid vehicle that contains a flux. The vehicle may also comprise an expendable organic binder to hold the powder in a self-sustaining mass following application. To form the connection, the board features a solder-wettable bond pad that constitutes a first faying surface. Similarly, the component includes a second, solder-wettable faying surface, for example, a contact. The solder paste is conveniently applied to the bond pad, for example, by screen printing, after which the component is arranged with the second faying surface in contact with the solder paste deposit. The assembly is then heated to vaporize the vehicle and to melt and reflow the solder alloy. Upon cooling, the solder alloy resolidifies and bonds to the faying surfaces to complete the connection. The solder connection not only physically attaches the component to the board, but also electrically connects the bond pad on the board and the contact of the component for conducting electrical current to or from the component for processing.
Common solder is formed of tin-lead alloy. It is proposed to form a lead-free solder connection composed of tin-bismuth alloy. However, tin-bismuth alloy tends to exhibit poor mechanical properties at elevated temperatures of the type encountered by microelectronic packages during use. In particular, such solder alloys tend to become unacceptably soft at temperatures as low as 100.degree. C. and exhibit a relatively low melting temperature, particularly in comparison to the common tin-lead solders.